leds: Let the binding document example for leds-gpio follow the gpio bindings

In the gpio bindings documents it is requested to use the marco's in
include/dt-bindings/gpio/gpio.h whenever possible. The gpios in the led
drivers don't seem to form an exception, so update the example in the
document bindings.

Signed-off-by: Olliver Schinagl <oliver@schinagl.nl>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Bryan Wu <cooloney@gmail.com>